The present invention is directed to a method for burning a fuel and a burner tip for burning of the fuel which permits suppression of generation of oxides of nitrogen (NOx).
Historically, the principal goals in burning a fuel for processes utilizing the heat of combustion were the operational goals of maintenance of stable combustion and maximum combustion efficiency and fuel utilization. More recently, due to environmental considerations, the emphasis has shifted to balancing those goals with the reduction of combustion emissions, especially particulate and NOx emissions. Substantial efforts have been and are being made to minimize and suppress particulate and NOx emissions when burning a fuel. Unfortunately, suppression of NOx emissions often results in failure to meet the operational goals of burner flame stability and efficient utilization of the fuel, as well as the goal of low particulate emissions.
To improve burner flame stability and combustion efficiency, intimate mixing between combustion air and fuel traditionally has been encouraged. However, in the development of the invention described herein, it has been determined that this intensified mixing contributes to a significant increase in NOx emission.
Attempts have been made to reconcile the conflicting goals of low NOx emission versus flame stability, high fuel utilization and efficiency, and low particulate emissions (smoke). U.S. Pat. Nos. 3,787,168 and 3,880,571 issued to Koppang et al and assigned to TRW, Inc. are directed to a solution of these conflicting goals.
These patents describe a burner hardware which controls NOx emission by reducing the contact time of the hot nitrogen molecules with atmospheric oxygen and conducting the combustion process at a low temperature. To achieve these objectives the invention teaches the use of improved fuel atomization and mixing of the reactants to complete the combustion rapidly and therefore reduce the residence time to a minimum for the reacting species that produce NOx. The reduction of flame temperature is accomplished by radiating heat away from the flame and by diluting the bulk of the reactants with an inert gas. The patents describe a distribution tube for radial introduction of a mixture of a fuel and a mixing gas into the combustion gas. A deflector disk is affixed to the end of the distribution tube to maximize the mixing intensity between the fuel and the oxidizer and to control the shape of the flame zone established. Another deflector may be suspended from the deflector disk for deflecting axially introduced cooling gas in a radial direction outwardly along the trajectory of the flame to carry away heat.
Because of increasingly strict regulations regarding NOx emissions from stationary sources, the assignee of the present invention contracted with TRW to apply the technology described in the aforementioned patents in its electrical generating plants. The burner tip configurations actually tested were later developments of what is described in the aforementioned patents.
The TRW burner technology was demonstrated in a utility boiler in tests conducted by SCE and TRW Research scientists and engineers. These tests showed that use of intense mixing for improved fuel atomization as taught by the TRW patents does not result in a low level of NOx emissions, but rather results in a high level of NOx emissions. It was also shown that completing the combustion process near homogeneous stoichiometric conditions, by intensifying the mixing process, as described in the TRW patents further increases NOx emissions. Burner features that were identified, after two years of extensive testing under the SCE-funded program to reduce NOx production, were described by D. B. Sheppard in "Low-NOx Burner Presentation", NOx Workshop and Assessment of Control Technology for Oil-and-Gas Fired Utility Boilers, October, 1977, Sigma Research, Inc., Richland Wash., pages 53-65. As reported in that paper, a new burner tip was used that did away with the deflector disk. In the new burner tip, atomizing steam was introduced through an annular passage surrounding a central oil passage. Oil flowed axially through the oil passage into a distribution chamber from which it was injected radially from orifices coaligned with steam orifices. The orifice pairs consisting of a steam orifice and an oil orifice were arranged circumferentially around the burner tip. Results of the studies with this type of burner tip are described in reports prepared by TRW entitled "Low NOx Burner Development Program", Final Report, January , 1976; "Low NOx Burner Development Program", Phase Two Final Report, June, 1976; and "Low NOx Burner Development Program", Phase Three Final Report, December, 1976. Each of these three reports accompanies this specification.
Another TRW suggested concept was the use of a two-stage burner gun where fuel is fed through two separate concentric annular passages, each terminating at laterally spaced apart orifices along the burner tip. The fuel is still atomized by gas introduced through coaligned orifices. Results obtained with this burner gun are reported in a report entitled "Low NOx Burner Development Program; Advanced LNB Program"; M1-J Test Facility Program, Aug. 25, 1978.
It was found that the various TRW burner tips were able to reduce NOx emissions from a utility boiler, without adversely affecting flame stability and fuel utilization efficiency. However, several deficiencies in the burner tip design were noted. For example, there was a tendency for carbon to accumulate around the steam orifices, thereby plugging the orifices and eventually the burner throat. In addition, large quantities of steam were required for atomization, in the range of 10-20% by weight of the oil flow rate. Furthermore, due to the use of concentric and aligned orifices, the burner tip assembly was bulky and heavy, which made it difficult to install and service. And finally, and most importantly, although there was a reduction in NOx as compared to the original equipment burner assemblies provided with the boiler, which used mechanical atomization and axial introduction of the oil, an even greater reduction in emissions was desired.
In view of the foregoing, it is apparent that there is a need for a method for burning fuel and a burner tip assembly which suppress NOx emissions, and which require minimal amounts of steam, where the burner tip prevents carbon deposits from forming in injection orifices. In addition, it is desirable that the burner tip be of small size and light weight.